Rubber-reinforcing cord and rubber product including same

ABSTRACT

A rubber-reinforcing cord (30) according to the present invention includes: a reinforcing fiber or reinforcing fiber bundle (21); a first coating (22) provided to cover the reinforcing fiber or reinforcing fiber bundle (21); and a second coating (31) provided to cover the first coating (22) and be located at the outer surface of the rubber-reinforcing cord (30). The first coating (22) includes: first rubber including nitrile-based rubber as a main component; and a first crosslinking agent. The second coating (31) is a coating different from the first coating (22) and includes: second rubber consisting essentially of nitrile-based rubber; and a second crosslinking agent. The nitrile-based rubber is at least one selected from nitrile rubber, hydrogenated nitrile rubber, carboxylated nitrile rubber, and carboxylated hydrogenated nitrile rubber.

TECHNICAL FIELD

The present invention relates to a rubber-reinforcing cord and a rubberproduct including the rubber-reinforcing cord.

BACKGROUND ART

A widely used approach for increasing the strength and durability of arubber product such as a toothed rubber belt is to embed a reinforcingcord including a reinforcing fiber such as a glass fiber or chemicalfiber in matrix rubber of the rubber product. The surface of thereinforcing fiber of the reinforcing cord is generally provided withcoatings for protecting the reinforcing fiber and for enhancing theadhesion between the matrix rubber and the reinforcing fiber.

It is commonly known that, when the matrix rubber includes hydrogenatednitrile rubber as a main component, a coating containingchlorosulfonated polyethylene (hereinafter referred to as “CSM”) as amain component is suitably used as one of the coatings of therubber-reinforcing cord that is provided at the outermost side (outersurface) of the rubber-reinforcing cord and is therefore to be incontact with the matrix rubber (see Patent Literature 1, for example).

CITATION LIST Patent Literature

-   Patent Literature 1: WO 2007/129624 A1

SUMMARY OF INVENTION Technical Problem

However, if a rubber-reinforcing cord having a CSM-containing coatinglocated to be in contact with matrix rubber of rubber products isemployed in a rubber product used in an environment involving exposureto an oil such as in the oil, the protection of the reinforcing fiber bythe coating will be insufficient, and the adhesion between the matrixrubber and the rubber-reinforcing cord will also be insufficient.Specifically, CSM is susceptible to oils, and oil particles can soeasily become trapped within and between the molecules of CSM thatexposure of the CSM-containing coating to an oil causes swelling of thecoating. The swelling results in separation of the coating from both thereinforcing fiber and the matrix rubber, leading to insufficientprotection of the reinforcing fiber by the coating and insufficientadhesion between the matrix rubber and the reinforcing fiber.

It is therefore an object of the present invention to provide arubber-reinforcing cord capable of ensuring satisfactory protection of areinforcing fiber by a coating and achieving strong adhesion to matrixrubber even when employed, for example, in a rubber product whose matrixrubber includes hydrogenated nitrile rubber and that is used in anenvironment involving exposure to an oil. Another object of the presentinvention is to provide a rubber product reinforced by such arubber-reinforcing cord and capable of maintaining high strength over along period of time.

Solution to Problem

The present invention provides a rubber-reinforcing cord for reinforcinga rubber product,

-   -   the rubber-reinforcing cord including:    -   a reinforcing fiber or a reinforcing fiber bundle;    -   a first coating provided to cover the reinforcing fiber or the        reinforcing fiber bundle; and    -   a second coating provided to cover the first coating and be        located at an outer surface of the rubber-reinforcing cord,        wherein    -   the first coating includes: first rubber including nitrile-based        rubber as a main component; and a first crosslinking agent,    -   the second coating is a coating different from the first coating        and includes: second rubber consisting essentially of        nitrile-based rubber; and a second crosslinking agent, and    -   the nitrile-based rubber is at least one selected from nitrile        rubber, hydrogenated nitrile rubber, carboxylated nitrile        rubber, and carboxylated hydrogenated nitrile rubber.

The present invention also provides a rubber product reinforced by theabove rubber-reinforcing cord according to the present invention.

Advantageous Effects of Invention

Even when employed in a rubber product whose matrix rubber includeshydrogenated nitrile rubber and that is used in an environment involvingexposure to an oil, the rubber-reinforcing cord according to the presentinvention is capable of ensuring satisfactory protection of thereinforcing fiber by the coatings and achieving strong adhesion to thematrix rubber. The rubber product according to the present invention,being reinforced by such a rubber-reinforcing cord, can maintain highstrength over a long period of time even when used in an environmentinvolving exposure to an oil.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial exploded perspective view schematically showing anexample of the rubber product according to the present invention.

FIG. 2 is a cross-sectional view schematically showing an exemplaryreinforcing fiber bundle (strand) included in the rubber-reinforcingcord according to the present invention and having the first coatingformed on its surface.

FIG. 3 is a cross-sectional view schematically showing an example of therubber-reinforcing cord according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described.

[Rubber-Reinforcing Cord]

A rubber-reinforcing cord according to the present embodiment is a cordfor reinforcing rubber products. This rubber-reinforcing cord includes:a reinforcing fiber or a reinforcing fiber bundle composed ofreinforcing fibers bundled together; a first coating provided to coverthe reinforcing fiber or the reinforcing fiber bundle; and a secondcoating provided to cover the first coating and be located at an outersurface of the rubber-reinforcing cord. The first coating includes:first rubber including nitrile-based rubber as a main component; and afirst crosslinking agent. The second coating is a coating different fromthe first coating and includes: second rubber consisting essentially ofnitrile-based rubber; and a second crosslinking agent. The nitrile-basedrubber as described herein is at least one selected from nitrile rubber(NBR), hydrogenated nitrile rubber (HNBR), carboxylated nitrile rubber(X-NBR), and carboxylated hydrogenated nitrile rubber (X-HNBR).

Hereinafter, a method for producing the reinforcing cord according tothe present embodiment will be described in detail.

(Reinforcing Fiber and Reinforcing Fiber Bundle)

The reinforcing fiber included in the rubber-reinforcing cord accordingto the present embodiment may be any fiber capable of enhancing theshape stability and strength of rubber products, and the material andshape of the fiber are not particularly limited.

Examples of fibers that can be used as the reinforcing fiber include: aglass fiber; a polyvinyl alcohol fiber typified by a vinylon fiber; apolyester fiber; a polyamide fiber such as nylon and aramid (aromaticpolyamide); a polyarylate fiber; a polyketone fiber; a carbon fiber; anda poly(p-phenylene benzoxazole) (PBO) fiber. Among these, a glass fiberis suitably used since it is superior in various properties such asdimensional stability, heat resistance, and tensile strength. Forexample, when a rubber-reinforcing cord for reinforcing a rubber productthat needs to have high durability is fabricated, it is preferable touse a glass fiber with adequate strength.

The type of the glass of the glass fiber is not particularly limited.High-strength glass, which has high tensile strength, is more preferredthan common alkali-free glass.

The structure of the reinforcing fiber is not particularly limited. Forexample, when the fiber is a glass fiber, it is preferable thatfilaments contained as the smallest structural components of the fiberhave an average diameter of 5 to 13 μm. For example, 50 to 2000 suchfilaments are formed into a bundle with the aid of a sizing agent, andone bundle thus formed is used as a reinforcing fiber or a plurality ofsuch bundles are aligned into a reinforcing fiber. A plurality of suchreinforcing fibers are aligned into a reinforcing fiber bundle.

The reinforcing fiber bundle may be formed, for example, by primarilytwisting the above reinforcing fiber, aligning a plurality of suchtwisted reinforcing fibers, and finally twisting the plurality of fibersin a direction opposite to the direction of the primary twisting.

The method for forming the reinforcing fiber bundle is not limited tothat described above and, for example, the following various types ofreinforcing fiber bundles can be employed: a reinforcing fiber bundleformed without the final twisting; a reinforcing fiber bundle formed byperforming the primary and final twisting in the same direction; areinforcing fiber bundle the central and peripheral portions of whichare constituted by different fibers made of different materials; and areinforcing fiber bundle in which the directions of the primary andfinal twisting vary. A reinforcing fiber bundle thus formed by twistingis suitable for use in a product, such as a reinforcing cord for atoothed belt, which needs to have bending resistance.

As described above, a reinforcing fiber bundle is formed by bundling aplurality of reinforcing fibers together. Reinforcing fibers whosesurface is coated with an undercoat layer may be prepared beforehand,and these reinforcing fibers may be used to form a reinforcing fiberbundle.

(First Coating)

The first coating includes: first rubber including nitrile-based rubberas a main component; and a first crosslinking agent. The first rubber asdescribed herein refers to the total rubber component contained in thefirst coating. Saying that the first coating includes first rubberincluding nitrile-based rubber as a main component therefore means thatthe nitrile-based rubber is contained in such an amount as to accountfor a major proportion of the total rubber component contained in thefirst coating. Saying that the first rubber includes a nitrile-basedrubber as a main component means that the content of the nitrile-basedrubber is 50 mass % or more with respect to the total rubber componentcontained in the first coating.

Nitrile-based rubber hardly absorbs oils and has high oil resistance.The fact that the first coating includes the first rubber including suchnitrile-based rubber as a main component enables the first coating, evenwhen the rubber-reinforcing cord according to the present embodiment isemployed in a rubber product used in an environment involving contactwith an oil, to maintain satisfactory protection of the reinforcingfiber or the reinforcing fiber bundle over a long period of time. Toimprove the function of the first coating as a protective layer, thefirst rubber preferably contains 60 mass % or more of nitrile-basedrubber and more preferably consists essentially of nitrile-based rubber.Saying that the first rubber consists essentially of nitrile-basedrubber means that the content of the nitrile-based rubber in the firstrubber is 80 mass % or more, preferably 90 mass % or more, and morepreferably 95 mass % or more. The first rubber may consist ofnitrile-based rubber.

The first rubber preferably includes HNBR as a main component. Sayingthat the first rubber includes HNBR as a main component means that thecontent of HNBR in the first rubber is 50 mass % or more. The firstrubber more preferably contains 60 mass % or more of HNBR and even morepreferably consists essentially of HNBR. Saying that the first rubberconsists essentially of HNBR means that the content of HNBR in the firstrubber is 80 mass % or more, preferably 90 mass % or more, and morepreferably 95 mass % or more. The first rubber may consist of HNBR.

The first coating includes the first crosslinking agent. This firstcrosslinking agent can cause the first rubber to be crosslinked in thefirst coating. In consequence, the reinforcing fibers or the reinforcingfiber bundles strongly adhere to each other via the first coating, andthe strength of the first coating itself is enhanced by the chemicalbond. This further improves the protective function of the first coatingfor the reinforcing fiber or reinforcing fiber bundle. When the secondcoating is provided in contact with the first coating, the crosslinkingstrengthens the adhesion at the interface between the first coating andthe second coating, thus leading to excellent protective function of thefirst coating and second coating for the reinforcing fiber orreinforcing fiber bundle and high adhesion strength between therubber-reinforcing cord and matrix rubber.

Examples of the first crosslinking agent include: quinone dioximecrosslinking agents such as p-quinone dioxime; methacrylate crosslinkingagents such as lauryl methacrylate and methyl methacrylate; allylcrosslinking agents such as DAF (diallyl fumarate), DAP (diallylphthalate), TAC (triallyl cyanurate), and TAIL (triallylisocyanurate);maleimide crosslinking agents such as bismaleimide, phenylmaleimide, andN,N′-m-phenylenedimaleimide; isocyanate compounds such as aromatic oraliphatic organic diisocyanates, polyisocyanate, blocked isocyanate, andblocked polyisocyanate; aromatic nitroso compounds; sulfur; andperoxides. These crosslinking agents may be used alone, or a pluralitythereof may be used in combination. The first crosslinking agent may beat least one selected from the group consisting of the maleimidecrosslinking agents and the isocyanate compounds. The maleimidecrosslinking agents or the isocyanate compounds, when used incombination with a rubber latex, can specifically enhance the adhesionbetween the first coating and another coating in contact with the firstcoating (the adhesion between the first coating and the second coatingin the case where the second coating is provided in contact with thefirst coating).

When the first crosslinking agent includes a maleimide crosslinkingagent, the content of the maleimide crosslinking agent in the firstcoating is, for example, 10 to 50 parts by mass, preferably 20 to 40parts by mass, more preferably 25 to 35 parts by mass, and particularlypreferably 27 to 33 parts by mass, with respect to 100 parts by mass ofthe first rubber.

Examples of preferred combinations of the first rubber and the firstcrosslinking agent in the first coating include a combination of thefirst rubber including HNBR and the first crosslinking agent including amaleimide crosslinking agent.

The total content of the first rubber and the first crosslinking agentin the first coating is, for example, 50 mass % or more. The firstcoating can be formed, for example, from a first water-based treatmentagent including the first rubber and the first crosslinking agent asmain components. The term “main components” is intended to mean that thetotal proportion of the components in the constituting components(excluding the solvent) of the first water-based treatment agent is 50mass % or more.

The first coating may further include a filler. The filler may be anorganic filler or an inorganic filler. Examples of the filler includecarbon black and silica. The addition of the filler can reduce theproduction cost of the rubber-reinforcing cord and can also effectivelyenhance the adhesion between the first coating and another coating incontact with the first coating (the adhesion between the first coatingand the second coating in the case where the second coating is providedin contact with the first coating). In particular, carbon black issuitably used since it can increase the cohesion of the treatment agent.To increase the adhesiveness or improve the stability, the first coatingmay include additives such as a plasticizer, anti-aging agent, metaloxide, and crosslinking aid.

The thickness of the first coating and the proportion of the firstcoating in the whole rubber-reinforcing cord are not particularlylimited, and may be adjusted appropriately depending on the propertiesrequired of the rubber-reinforcing cord or the type of the reinforcingfiber. In an example, the proportion of the first coating in the wholerubber-reinforcing cord is in the range of 5 to 30 mass %, preferably inthe range of 10 to 25 mass %, and more preferably in the range of 13 to19 mass %.

The first coating only has to be provided to cover the reinforcing fiberor the reinforcing fiber bundle. The first coating may be provided incontact with the reinforcing fiber or the reinforcing fiber bundle.Alternatively, another coating such as an undercoat layer may beprovided on the reinforcing fiber or the reinforcing fiber bundle, andthe first coating may be provided to cover the reinforcing fiber or thereinforcing fiber bundle with the undercoat layer interposedtherebetween.

(Second Coating)

The second coating includes: second rubber consisting essentially ofnitrile-based rubber; and a second crosslinking agent. The second rubberas described herein refers to the total rubber component contained inthe second coating. Saying that the second coating includes secondrubber consisting essentially of nitrile-based rubber therefore meansthat the total rubber component contained in the second coating consistsessentially of nitrile-based rubber. Saying that the second rubberconsists essentially of nitrile-based rubber means that the proportionof the nitrile-based rubber in the total rubber component contained inthe second coating is 80 mass % or more, preferably 90 mass % or more,and more preferably 95 mass % or more. The second rubber may consist ofnitrile-based rubber.

As previously described, nitrile-based rubber hardly absorbs oils andhas high oil resistance. The fact that the rubber component contained inthe second coating provided at the outer surface of therubber-reinforcing cord consists essentially of nitrile-based rubbertherefore enables the second coating, even when the rubber-reinforcingcord according to the present embodiment is employed in a rubber productused in an environment involving contact with an oil, to maintainsatisfactory protection of the reinforcing fiber or the reinforcingfiber bundle over a long period of time and achieve strong adhesion tothe matrix rubber of the rubber product.

Additionally, since the rubber component contained in the second coatingconsists essentially of nitrile-based rubber, the formation of thesecond coating does not require the use of an organic solvent such asxylene for application of a treatment agent for forming the coating,unlike the formation of conventional coatings including CSM as a maincomponent. The second coating thus has the advantage of being able to beformed using a water-based treatment agent.

The second rubber preferably includes HNBR as a main component. Sayingthat the second rubber includes HNBR as a main component means that thecontent of HNBR in the second rubber is 50 mass % or more. The secondrubber more preferably contains 60 mass % or more of HNBR and morepreferably consists essentially of HNBR. Saying that the second rubberconsists essentially of HNBR means that the content of HNBR in thesecond rubber is 80 mass % or more, preferably 90 mass % or more, andmore preferably 95 mass % or more. The second rubber may consist ofHNBR.

The second coating includes the second crosslinking agent. This secondcrosslinking agent can cause the second rubber to be crosslinked in thesecond coating. In consequence, the rubber-reinforcing cord stronglyadheres to matrix rubber, and the strength of the second coating itselfis enhanced by the chemical bond. When the second coating is provided incontact with the first coating, the crosslinking strengthens theadhesion at the interface between the first coating and the secondcoating, thus leading to excellent protective function of the firstcoating and second coating for the reinforcing fiber or reinforcingfiber bundle and high adhesion strength between the rubber-reinforcingcord and matrix rubber.

The crosslinking agents mentioned as examples of the first crosslinkingagent can be used as the second crosslinking agent. As is the case withthe first crosslinking agent, the crosslinking agents may be used alone,or a plurality thereof may be used in combination. The secondcrosslinking agent may be at least one selected from the groupconsisting of maleimide crosslinking agents and isocyanate compounds.The maleimide crosslinking agents or the isocyanate compounds, when usedin combination with the second rubber consisting essentially ofnitrile-based rubber, can specifically enhance the adhesion between thesecond coating and the matrix rubber.

When the second crosslinking agent includes a maleimide crosslinkingagent, the content of the maleimide crosslinking agent in the secondcoating is, for example, 10 to 50 parts by mass, preferably 20 to 40parts by mass, more preferably 25 to 35 parts by mass, and particularlypreferably 27 to 33 parts by mass, with respect to 100 parts by mass ofthe second rubber. When the second crosslinking agent includes anisocyanate compound, the content of the isocyanate compound in thesecond coating is, for example, 10 to 50 parts by mass, preferably 20 to40 parts by mass, more preferably 25 to 35 parts by mass, andparticularly preferably 27 to 33 parts by mass, with respect to 100parts by mass of the second rubber. When the second coating contains amaleimide crosslinking agent and/or isocyanate compound as the secondcrosslinking agent in an amount within the above range, the adhesionstrength between the rubber-reinforcing cord and a rubber latex can befurther enhanced.

When the second crosslinking agent includes both a maleimidecrosslinking agent and an isocyanate compound, the respective contentsof the maleimide crosslinking agent and the isocyanate compound are, forexample, 10 to 50 parts by mass, preferably 20 to 40 parts by mass, morepreferably 25 to 35 parts by mass, and particularly preferably 27 to 33parts by mass, with respect to 100 parts by mass of the second rubber.

Examples of preferred combinations of the second rubber and the secondcrosslinking agent in the second coating include a combination of thesecond rubber including HNBR and the second crosslinking agent includinga maleimide crosslinking agent and an isocyanate compound.

The total content of the second rubber and the second crosslinking agentin the second coating is, for example, 50 mass % or more. The secondcoating can be formed from a second water-based treatment agentincluding the second rubber and the second crosslinking agent as maincomponents. The term “main components” is intended to mean that thetotal proportion of the components in the constituting components(excluding the solvent) of the second water-based treatment agent is 50mass % or more.

The second coating may further include a filler. The filler may be anorganic filler or an inorganic filler. Examples of the filler includecarbon black and silica. The addition of the filler can reduce theproduction cost of the rubber-reinforcing cord and can effectivelyenhance the adhesion between the rubber-reinforcing cord and matrixrubber. In particular, carbon black is suitably used since it canincrease the cohesion of the treatment agent. When the second coatingincludes carbon black, the content of the carbon black in the secondcoating is, for example, 2 to 20 parts by mass, preferably 5 to 17 partsby mass, and more preferably 7 to 15 parts by mass, with respect to 100parts by mass of the second rubber.

The total content of the above three components (maleimide crosslinkingagent, isocyanate compound, and carbon black) in the second coating ispreferably 100 parts by mass or less with respect to 100 parts by massof the second rubber. Adjusting the total content of the threecomponents to 100 parts by mass or less makes the formation of thecoating easy, thus leading to enhancement of the protective function ofthe coating for the reinforcing fiber or reinforcing fiber bundle and toimprovement in the bending fatigue resistance of the rubber-reinforcingcord.

To increase the adhesiveness or improve the stability, the secondcoating may further include additives such as a plasticizer, anti-agingagent, metal oxide, and crosslinking aid.

The thickness of the second coating and the proportion of the secondcoating in the whole rubber-reinforcing cord are not particularlylimited, and may be adjusted appropriately depending on the propertiesrequired of the rubber-reinforcing cord or the type of the reinforcingfiber. In an example, the proportion of the second coating in the wholerubber-reinforcing cord is in the range of 0.5 to 10 mass %, preferablyin the range of 1 to 7 mass %, and more preferably in the range of 1.2to 5 mass %.

The second coating is a coating different from the first coating. Thecomponents of the second coating and the ratio between the components ofthe second coating are therefore not exactly identical to the componentsof the first coating and the ratio between the components of the firstcoating. The second coating only has to be provided to be located at theouter surface of the rubber-reinforcing cord; namely, the second coatingonly has to be provided to be located at the outermost side of therubber-reinforcing cord so that the surface of the second coating isexposed to the outside and that the second coating covers the firstcoating. This means that the second coating need not be in contact withthe first coating. For example, another coating may be provided betweenthe first coating and the second coating, and the second coating maycover the first coating with the other coating interposed therebetween.However, the provision of the second coating in contact with the firstcoating leads to strong bonding between the coatings and hence toincreased overall strength of the coatings, since both the first coatingand the second coating include nitrile-based rubber as a rubbercomponent and further include a crosslinking agent. Furthermore, whenthe first and second coatings, which include nitrile-based rubber whichhas high oil resistance, are provided without any other coatinginterposed therebetween, the overall oil resistance of the coatings isfurther increased. For these reasons, more satisfactory protection ofthe reinforcing fiber by the first and second coatings and strongadhesion to matrix rubber can be achieved.

Example of Method for Producing Rubber-Reinforcing Cord

Hereinafter, an example of the method for producing therubber-reinforcing cord according to the present embodiment will bedescribed. First, the first coating is formed to cover a reinforcingfiber or a reinforcing fiber bundle. The reinforcing fiber orreinforcing fiber bundle to be prepared is as described above. The firstcoating can be formed from the first water-based treatment agentincluding the first rubber and the first crosslinking agent as maincomponents. Specifically, the first water-based treatment agent isapplied to the reinforcing fiber or reinforcing fiber bundle and thendried to form the first coating. The first water-based treatment agentincludes components for forming the first coating (the first rubber, thefirst crosslinking agent, and an additive such as a filler which may beadded as necessary) and a solvent. The application of the firstwater-based treatment agent is followed by removal of the solvent,resulting in the formation of the first coating. For example, water isused as the solvent. Thus, a treatment agent prepared by dissolving ordispersing a latex of the first rubber and the first crosslinking agentin water may be used as the first water-based treatment agent. A filler,plasticizer, anti-aging agent, metal oxide, and crosslinking aid may, ifdesired, be added to the treatment agent to increase the adhesiveness ofthe coating or improve the stability of the coating.

The method for applying and drying the first water-based treatment agentis not particularly limited. Typically, the reinforcing fiber orreinforcing fiber bundle is immersed in a bath containing the firstwater-based treatment agent, and then dried in a drying oven to removethe solvent and thus form the first coating. The conditions of thedrying for removing the solvent are not particularly limited; however,it is necessary to avoid carrying out the drying under conditions wherethe reaction of the crosslinking agent in the coating can proceed tocompletion. It is therefore preferable to shorten the drying time (to 5minutes or less, for example) when performing the drying at a relativelyhigh temperature (at 80° C. or above, for example). When, for example,the temperature of the atmosphere is 150° C. or below, the drying timemay be 5 minutes or less. In an example, the drying may be performed inan atmosphere at 80° C. to 280° C. for 0.1 to 2 minutes.

A plurality of reinforcing fibers or reinforcing fiber bundles eachhaving the first coating formed thereon may be prepared and twistedtogether. This allows the reinforcing fibers or reinforcing fiberbundles to come into close contact with each other via the firstcoatings. At this stage, the reaction of the first crosslinking agenthas yet to take place, and the first coatings are flexible and in closecontact with each other so that the reinforcing fibers or reinforcingfiber bundles can sufficiently strongly adhere to each other.

The second coating is then formed when the second coating is to beprovided in contact with the first coating. As described above, thesecond coating can be formed from the second water-based treatment agentincluding the second rubber and the second crosslinking agent as maincomponents. Specifically, the second water-based treatment agent isapplied onto the first coating and then dried to form the secondcoating. The second water-based treatment agent includes components forforming the second coating (the second rubber, the second crosslinkingagent, and an additive such as a filler which may be added as necessary)and a solvent. The application of the second water-based treatment agentis followed by removal of the solvent, resulting in the formation of thesecond coating. For example, water is used as the solvent. Thus, atreatment agent prepared by dissolving or dispersing a latex of thesecond rubber and the second crosslinking agent in water may be used asthe second water-based treatment agent. A filler, plasticizer,anti-aging agent, metal oxide, and crosslinking aid may, if desired, beadded to the treatment agent to increase the adhesiveness of the coatingor improve the stability of the coating.

The method for applying and drying the second water-based treatmentagent is not particularly limited. Typically, the reinforcing fiber orreinforcing fiber bundle is immersed in a bath containing the secondwater-based treatment agent, and then dried in a drying oven to removethe solvent and thus form the second coating. The conditions of thedrying for removing the solvent are not particularly limited; however,it is necessary to avoid carrying out the drying under conditions wherethe reaction of the crosslinking agent in the coating can proceed tocompletion. It is therefore preferable to shorten the drying time (to 5minutes or less, for example) when performing the drying at a relativelyhigh temperature (at 80° C. or above, for example). When, for example,the temperature of the atmosphere is 150° C. or below, the drying timemay be 5 minutes or less. In an example, the drying may be performed inan atmosphere at 80° C. to 280° C. for 0.1 to 2 minutes.

Another coating can be further formed between the reinforcing fiber orreinforcing fiber bundle and the first coating and/or between the firstcoating and the second coating.

A heat treatment for inducing crosslinking of the rubber componentscontained in the first coating and second coating may be performedbefore or after embedding of the rubber-reinforcing cord in a rubberproduct.

[Rubber Product]

A rubber product according to the present embodiment will be described.The rubber product according to the present embodiment includes matrixrubber and a rubber-reinforcing cord embedded in the matrix rubber. Therubber-reinforcing cord used is the above rubber-reinforcing cordaccording to the present embodiment.

The technique for embedding the rubber-reinforcing cord in the matrixrubber of the rubber product is not particularly limited, and a knowntechnique can be employed. When crosslinking of the rubber-reinforcingcord according to the present embodiment is allowed to take placesimultaneously with crosslinking of the rubber product, the unitybetween the rubber-reinforcing cord and the rubber product can beenhanced, and the steps of production of the rubber-reinforcing cord canbe simplified. The rubber product thus obtained has high heat resistanceattributed to the properties of the matrix rubber and further has highstrength and high bending fatigue resistance which result from theembedding of the rubber-reinforcing cord. The rubber product istherefore applicable to various uses and is particularly suitable foruse in, for example, a timing belt of a vehicle engine.

The rubber product according to the present embodiment is produced, forexample, as follows. First, the rubber-reinforcing cord according to thepresent embodiment is prepared (any heat treatment that can causecrosslinking involving the crosslinking agents in the coatings toproceed fully should be avoided). Next, the rubber-reinforcing cord isembedded in matrix rubber including rubber and a crosslinking agent. Thematrix rubber with the embedded rubber-reinforcing cord is thenheat-treated under conditions where the reaction of the crosslinkingagents proceed fully, and thus the rubber of each coating of therubber-reinforcing cord and the matrix rubber of the rubber product aresimultaneously crosslinked.

An example of the rubber product according to the present embodimentwill be described. FIG. 1 shows an exploded perspective view of atoothed belt 10. The toothed belt 10 includes matrix rubber 11 andrubber-reinforcing cords 12 embedded in the matrix rubber 11. The matrixrubber 11 is composed of rubber or composed of rubber and anothermaterial. The rubber-reinforcing cords 12 are each therubber-reinforcing cord according to the present embodiment, and arearranged parallel to the moving direction of the toothed belt 10. Knownproducts can be used as the parts of the belt other than therubber-reinforcing cords 12.

EXAMPLES

Hereinafter, the embodiment of the present invention will be describedin more detail with Examples and Comparative Examples.

Example 1

<Production of Rubber-Reinforcing Cord>

A glass fiber (reinforcing fiber) constituted by a bundle of 200 glassfilaments (composed of E-glass and having an average diameter of 9 μm)was prepared. Three such glass fibers were aligned into a bundle(reinforcing fiber bundle), and a water-based treatment agent (firstwater-based treatment agent) as shown in Table 1 was applied to thebundle and dried in a drying oven set at 150° C. for 1 minute. A strandwas thus formed.

FIG. 2 schematically shows a cross-section of a strand 20 formed asabove. Glass fibers 21 are each composed of a large number of filaments.A first coating 22 is formed to cover the surface of the bundle of thethree glass fibers 21. The three glass fibers 21 are adhered together bythe first coating 22. At this stage, the reaction of the crosslinkingagent in the first coating has yet to take place, and the components ofthe first coating are as shown in Table 1.

The strand obtained as above was primarily twisted at two turns/25 mm.Next, 11 such twisted strands were aligned and finally twisted at twoturns/25 mm. The proportion of the coating in the resulting cord was 20mass %.

A water-based treatment agent (second water-based treatment agent) ofExample 1 as shown in Table 2-1 was applied to the cord to form a secondcoating, which was dried in a drying oven at 150° C. for 1 minute. Arubber-reinforcing cord of Example 1 was thus obtained. FIG. 3schematically shows a cross-section of a cord 30 thus formed. Thestrands 20 are adhered to each other by the first coatings 22. A secondcoating 31 is formed at the surface of the cord 30. At this stage, thereaction of the crosslinking agent in the second coating has yet to takeplace, and the components of the second coating are as shown in Table2-1.

The rubber-reinforcing cord of Example 1 obtained as above was subjectedto the following evaluation tests.

<Adhesion Strength to Matrix Rubber and Type of Failure>

First, two pieces of rubber composed as shown in Table 3 (each piecehaving a width of 15 mm, a length of 50 mm, and a thickness of 5 mm)were prepared. Next, the rubber-reinforcing cord was sandwiched by thetwo rubber pieces in such a manner as to extend parallel to the lengthof the rubber pieces, and the cord and rubber pieces were adheredtogether by heating them at 150° C. for 20 minutes. The specimen thusprepared was pulled lengthwise by a tensile tester to measure the peelstrength between the matrix rubber and the rubber-reinforcing cord ofExample 1. It was also examined whether the failure of the specimen was“rubber failure” which occurred while the rubber-reinforcing cord andthe matrix rubber remained adhered, “interfacial peeling” which occurredat the interface between the matrix rubber and the rubber-reinforcingcord, or “spot” which is intermediate between “rubber failure” and“interfacial peeling”. Specifically, the “rubber failure” refers to atype of failure in which a crack appears within the matrix rubberwithout peeling at the interface between the matrix rubber and therubber-reinforcing cord and in which 90% or more of the surface of therubber-reinforcing cord is covered by the matrix rubber at the peelinginterface. The “spot” refers to a state where 20% or more and less than90% of the surface of the rubber-reinforcing cord is covered by thematrix rubber at the peeling interface. The “interfacial peeling” refersto a type of failure in which peeling occurs between the matrix rubberand the rubber-reinforcing cord without rubber failure and in which thepercentage of broken rubber present at the surface of the peeledrubber-reinforcing cord is less than 20%. The percentage of rubberpresent at the peeling interface was determined using a printed image ofa photograph of the peeling interface. Specifically, first, a photographof the specimen is taken so as to include the whole peeling interface, aportion corresponding to the whole specimen is cut out from a printedimage of the photograph, and the weight W of the cut-out printed imageof the whole specimen is measured. Next, a portion corresponding to therubber is cut out from the printed image of the whole specimen, and theweight w of the whole cut-out portion corresponding to the rubber ismeasured. Using the measured values of the weights W and w, thepercentage of the remaining rubber is determined ((w/W)×100%). Theresult is shown in Table 2-1.

<Oil Absorbency>

The treatment agent of Example 1 shown in Table 2-1 was poured into aTeflon (registered trademark) mold having a 30 cm×30 cm squarecross-section and a depth of 5 mm and was dried at 80° C. for 10 hours(first drying) to obtain a film. This film was subjected to seconddrying at 120° C. for 150 minutes, and the resulting film wascrosslinked at a temperature of 160° C. and a pressure of 100 kg/cm² for20 minutes. The resulting film was cut to prepare a dumbbell specimen(No. 3 dumbbell specimen as specified in JIS K 6251). This specimen wasimmersed in an engine oil (Castrol EDGE 5W-30) and allowed to stand inan oven at 120° C. for 24 hours. After that, the film was taken out,then the oil was thoroughly wiped off with a waste cloth, and the changein weight of the film was determined. This weight change was calculatedas the oil absorbency by the equation given below. The result is shownin Table 2-1.

Oil absorbency (%)={(Weight after oil immersion)−(Weight before oilimmersion)}/(Weight after oil immersion)×100

Examples 2 to 12 and Comparative Examples 1 to 12

<Production of Rubber-Reinforcing Cord>

Rubber-reinforcing cords of Examples 2 to 12 and Comparative Examples 1to 12 were produced in the same manner as in Example 1, except forapplying treatment agents of Examples 2 to 12 shown in Table 2-1 andtreatment agents of Comparative Examples 1 to 12 shown in Table 2-2instead of the treatment agent of Example 1.

<Adhesion Strength to Matrix Rubber and Peeling Interface>

Specimens of Examples 2 to 12 and Comparative Examples 1 to 12 wereprepared in the same manner as in Example 1, except for using therubber-reinforcing cords of Examples 2 to 12 and Comparative Examples 1to 12 instead of the rubber-reinforcing cord of Example 1. The preparedspecimens were tested in the same manner as in Example 1 to evaluate theadhesion strength and peeling interface. The results are shown in Tables2-1 and 2-2.

<Oil Absorbency>

Specimens of Examples 2 to 12 and Comparative Examples 1 to 12 wereprepared in the same manner as in Example 1, except for using thetreatment agents of Examples 2 to 12 shown in Table 2-1 and thetreatment agents of Comparative Examples 1 to 12 shown in Table 2-2instead of the treatment agent of Example 1. The prepared specimens weretested in the same manner as in Example 1 to evaluate the oilabsorbency. The results are shown in Tables 2-1 and 2-2.

TABLE 1 Components Proportions H-NBR latex (solid content: 40 mass %)(*1) 100 parts by mass  Aqueous bismaleimide dispersion 25 parts by mass(solid content: 50 mass %) Carbon black (solid content: 30 mass %) 35parts by mass (*1) Zetpol Latex (manufactured by Zeon Corporation)

TABLE 2-1 Examples 1 2 3 4 5 6 7 8 9 10 11 12 Components H-NBR latex 10050 50 100 100 100 100 of treatment (solid content: agents 40 mass %)(*1) (parts by X-NBR latex 100 50 50 100 mass of (solid content: solids)40 mass %) (*2) NBR latex 100 50 50 100 (solid content: 40 mass %) (*3)Aqueous 30 30 30 30 30 30 10 10 10 50 50 40 bismaleimide dispersion(solid content: 50 mass %) Blocked 25 25 25 25 25 25 10 10 10 40 30 40isocyanate (solid content: 40 mass %) Carbon black 10 10 10 10 10 10 2 22 10 20 20 (solid content: 30 mass %) Oil absorbency (%) 2.2 1.8 0.7 2.21.4 2.5 2.9 4.3 3.9 1.7 1.8 0.0 Adhesion strength (N/15 mm) 233.5 215.2222.2 234.2 228.2 258.4 188.7 154.5 204.9 275.6 185.1 183.4 Type offailure Rubber Rubber Rubber Rubber Rubber Rubber Rubber Rubber RubberRubber Rubber Rubber failure failure failure failure failure failurefailure failure failure failure failure failure (*1) Zetpol Latex(manufactured by Zeon Corporation) (*2) Nipol 1571H Latex (manufacturedby Zeon Corporation) (*3) Nipol 1562 Latex (manufactured by ZeonCorporation)

TABLE 2-2 Comparative Examples 1 2 3 4 5 6 7 8 9 10 11 12 Components CSMlatex 100 100 100 of treatment (solid content: agents 40 mass %) (*4)(parts by SBR latex 100 100 100 mass of (solid content: solids) 50 mass%) (*5) CR latex 100 100 100 (solid content: 50 mass %) (*6) VP latex100 100 100 (solid content: 40 mass %) (*7) Aqueous 30 10 50 30 10 50 3010 50 30 10 50 bismaleimide dispersion (solid content: 50 mass %)Blocked 25 10 40 25 10 40 25 10 40 25 10 40 isocyanate (solid content:40 mass %) Carbon black 10 2 20 10 2 20 10 2 20 10 2 20 (solid content:30 mass %) Oil absorbency (%) 30.3 35.2 18.9 13.0 31.0 7.7 5.3 5.5 5.330.3 35.2 35.0 Adhesion strength (N/15 mm) 52.3 85.8 47.0 47.5 47.1 41.115.3 141.1 94.6 60.1 39.7 84.5 Type of failure Inter- Inter- Inter-Inter- Inter- Inter- Spot Spot Spot Inter- Inter- Spot facial facialfacial facial facial facial facial facial peeling peeling peelingpeeling peeling peeling peeling peeling (*4) SEPOLEX CSM Latex(manufactured by Sumitomo Seika Chemicals Co., Ltd.), CSM:Chlorosulfonated polyethylene (*5) SB Latex (manufactured by Asahi KaseiChemicals Corporation), SB: Styrene-butadiene (*6) CR Latex(manufactured by Denki Kagaku Kogyo K.K.), CR: Chloroprene rubber (*7)PYRATEX (manufactured by Nippon A & L Inc.), VP:Vinylpyridine-styrene-butadiene

TABLE 3 Components Mass ratio Hydrogenated nitrile rubber (*1) 70 Zincdimethacrylate-containing hydrogenated nitrile 30 rubber (*2) Zinc oxide10 Stearic acid 1 Carbon black 30 Trioctyl trimellitate 5 Sulfur 0.11,3-bis(t-butylperoxy-isopropyl)benzene 6 Magnesium oxide 14,4′-(α,α-dimethylbenzyl)diphenylamine 0.5 2-mercaptobenzimidazole zincsalt 0.5 Triallyl isocyanurate 1 (*1) Zetpol 2020 (manufactured by ZeonCorporation) (*2) Zetpol 2000L (manufactured by Zeon Corporation)

The rubber-reinforcing cords of Examples 1 to 12, in each of whichnitrile-based rubber was used as the rubber composing the second coatinglocated at the outer surface, yielded lower oil absorbency than therubber-reinforcing cords of Comparative Examples 1 to 12 in which othertypes of rubber such as CSM were used as the rubber composing the secondcoatings. Furthermore, the rubber-reinforcing cords of Examples 1 to 12exhibited stronger adhesion to matrix rubber than the rubber-reinforcingcords of Comparative Examples 1 to 12. These results have confirmed thatthe rubber-reinforcing cord according to the present invention iscapable, even when employed in a rubber product used in an environmentinvolving contact with an oil, of ensuring satisfactory protection ofthe reinforcing fibers by the coatings and achieving strong adhesion tothe matrix rubber of the rubber product.

INDUSTRIAL APPLICABILITY

The present invention is applicable to various rubber products and torubber-reinforcing cords used in the rubber products. In particular, thepresent invention can be advantageously embodied as a rubber productused in an environment involving contact with an oil and as arubber-reinforcing cord employed in such a rubber product.

1. A rubber-reinforcing cord for reinforcing a rubber product, therubber-reinforcing cord comprising: a reinforcing fiber or a reinforcingfiber bundle; a first coating provided to cover the reinforcing fiber orthe reinforcing fiber bundle; and a second coating provided to cover thefirst coating and be located at an outer surface of therubber-reinforcing cord, wherein the first coating comprises: firstrubber comprising nitrile-based rubber as a main component; and a firstcrosslinking agent, the second coating is a coating different from thefirst coating and comprises: second rubber consisting essentially ofnitrile-based rubber; and a second crosslinking agent, and thenitrile-based rubber is at least one selected from nitrile rubber,hydrogenated nitrile rubber, carboxylated nitrile rubber, andcarboxylated hydrogenated nitrile rubber.
 2. The rubber-reinforcing cordaccording to claim 1, wherein the first rubber consists essentially ofthe nitrile-based rubber.
 3. The rubber-reinforcing cord according toclaim 1, wherein the first coating is a coating formed by drying a firstwater-based treatment agent comprising the first rubber and the firstcrosslinking agent as main components.
 4. The rubber-reinforcing cordaccording to claim 1, wherein the second coating is a coating formed bydrying a second water-based treatment agent comprising the second rubberand the second crosslinking agent as main components.
 5. Therubber-reinforcing cord according to claim 1, wherein the secondcrosslinking agent comprises at least one selected from a maleimidecrosslinking agent and an isocyanate compound.
 6. The rubber-reinforcingcord according to claim 5, wherein the second crosslinking agentcomprises the maleimide crosslinking agent, and the content of themaleimide crosslinking agent in the second coating is 10 to 50 parts bymass with respect to 100 parts by mass of the second rubber.
 7. Therubber-reinforcing cord according to claim 5, wherein the secondcrosslinking agent comprises the isocyanate compound, and the content ofthe isocyanate compound in the second coating is 10 to 50 parts by masswith respect to 100 parts by mass of the second rubber.
 8. A rubberproduct reinforced by the rubber-reinforcing cord according to claim 1.9. The rubber product according to claim 8, being a rubber beltcomprising matrix rubber and the rubber-reinforcing cord embedded in thematrix rubber.